An image duplicating apparatus is typical of an image forming apparatus of the nature to which the present invention generally appertains. An image forming apparatus of this type may be equipped with two or more developing units which are to be selectively put to use. These developing units have stocks of developer powders or agents of different natures such as different colors. For each cycle or a series of cycles of duplicating operation, any one of the developing units is selected for use to form images of, typically, different colors on a single or a plurality of print sheets.
In the meantime, it is known in the art that, in scanning a document sheet with a beam of light for duplicating the images thereon, it is preferable that the intensity of light be regulated depending on the nature of the developer agent to be used for the printing of the images reproduced. Thus, the user of a known image duplicating apparatus of the type using two or more developing units has been required to adjust the intensity of image scanning light depending on the nature of the developing agent stored in the selected developing unit. Laborious and time-taking steps must be taken by the operator of the apparatus for such adjustment of the intensity of light or, "exposure value" with which a document sheet is to be irradiated with a scanning beam of light. Such steps are liable to invite errors and may thus lead to failures of the apparatus or to reproduction of unwanted images from the original. These problems are frequently experienced in an image duplicating apparatus but will be also encounters in an image forming apparatus of another type or nature. The present invention contemplates elimination of such problems inherent in a prior-art image forming apparatus of the type using two or more developing units.